In the present experiments, an attempt was made to use [¹⁴C]adenine ([¹⁴C]A) for autoradiographic labeling of blood monocytes and their precursor cells (monocytic cells). Main results are as follows.
Following 3 daily injections of [¹⁴C]A (1μCi/g body weight each) into young adult rats, from 27 to 75% of blood monocytes together with the majority of their precursor cells in the bone marrow became labeled. Moreover, a portion (up to 7%) of the labeled blood monocytes had as many silver grains per cell as those of their precursor cells in the bone marrow, such as promonocytes or monoblasts.
The former findings, particularly the labeling indices of blood monocytes in the case of [¹⁴C]A labeling are almost the same as those in the case of [³H]TdR labeling reported by other workers. However, the latter finding, that is, the abundant occurrence of immature monocytes comparable to promonocytes or monoblasts in the circulating blood has not hitherto been reported by any worker who used [³H]TdR. This indicates that [¹⁴C]A is more effective for labeling monocytic cells than is [³H]TdR.
By treatments with RNase or DNase or both, it was confirmed that the observed labeling of monocytic cells with [¹⁴C]A was chiefly due to the incorporation of labeled adenine into DNA.
From the biochemical view point, it was suggested that monocytic cells have a limited capacity for de novo adenine synthesis and because of this [¹⁴C]A is especially effective for labeling these cells.